Multilayer coating film-forming methods for automobile bodies or automobile parts that are known in the prior art include three-coat, two-bake systems comprising (i) a step of coating an article to be coated (electrodeposited steel sheet, plastic or the like) with a primer coating (intercoat material) and heating it to cure the formed uncured primer coating film, (ii) a step of coating the primer coating film with a base coat material and forming an uncured base coating film, (iii) a step of coating the uncured base coating film with a clear coating material and forming an uncured clear coating film, and (iv) a step of heating to cure the uncured base coating film and the uncured clear coating film.
There is also widely known, as a multilayer coating film-forming method for automobile bodies or automobile parts, a two-coat, two-bake system comprising (i) a step of coating an article to be coated with a primer coating, and heating it to cure the formed uncured primer coating film, (ii) a step of coating the primer coating film with a top coat material and forming an uncured top coating film, and (iii) heating it to cure the uncured top coating film (optionally including a step of preheating after coating of the coating material).
Generally speaking, the three-coat, two-bake system is employed when a “metallic color” coating film is to be formed using a base coat material containing a luster pigment, while the two-coat, two-bake system is employed when a “solid color”, such as white or black coating film is to be formed using a top coat material containing a color pigment.
On the other hand, from the viewpoint of shortening the line steps and achieving energy savings in recent years, research has been carried out on 3-coat, 1-bake systems that eliminate the heating step after coating of the primer coating, comprising (i) a step of coating an article to be coated with a primer coating and forming an uncured primer coating film, (ii) a step of coating the uncured primer coating film with a base coat material and forming an uncured base coating film, (iii) a step of coating the uncured base coating film with a clear coating material and forming an uncured clear coating film, and (iv) a step of heating to cure the layered coating films.
Research has also been carried out on 2-coat, 1-bake systems comprising (i) a step of coating an article to be coated with a primer coating and forming an uncured primer coating film, (ii) a step of coating the uncured primer coating film with a top coat material and forming an uncured top coating film, and (iii) a step of heating to cure the layered coating films, and 2-coat, 1-bake systems comprising (i) a step of coating an article to be coated with a base coat material and forming an uncured base coating film, (ii) coating the uncured base coating film with a clear coating material and forming an uncured clear coating film, and (iii) heating to cure the layered coating film (optionally including a step of preheating after coating of the coating material). From the viewpoint of minimizing environmental pollution by volatilization of organic solvents in 3-coat, 1-bake systems and 2-coat, 1-bake systems, there is particular demand for systems employing aqueous coating materials as the base coat material and top coat material.
In these 3-coat, 1-bake systems and 2-coat, 1-bake systems, however, mixing of the coating films takes place between the aqueous base coating film (aqueous top coating film) and the primer coating film or between the aqueous base coating film (aqueous top coating film) and the clear coating film, often reducing the smoothness and distinctness of image of the multilayer coating film that is formed.
In addition, in a base coat material (top coat material) that requires a variety of types for different coating colors, it is possible to reduce costs and improve color matching by using the same for automobile body coating materials and automobile part coating materials. On the other hand, considering the heat resistance (and/or energy efficiency) of plastics as automobile part materials, there has been a desire for coating materials and multilayer coating film-forming methods that allow curing to be accomplished at lower temperatures than the conventional range of 120° C. to 160° C.
However, with conventional aqueous base coat materials (aqueous top coat materials) and multilayer coating film-forming methods, heating at low temperature results in insufficient curing of the multilayer coating film and the adhesion and water resistance of the multilayer coating film has sometimes been reduced.
In PTL 1, for example, there is described a 3-coat, 1-bake type of method of forming a coating film, in which an intercoating film, a base coating film and a clear coating film are formed in that order on a base material in a wet-on-wet manner, wherein the intercoat material forming the intercoating film and the base coating material forming the base coating film comprise an amide group-containing acrylic resin and a curing agent, and the curing agent in the intercoat material comprises an aliphatic isocyanate-based active methylene blocked isocyanate.
PTL 1 teaches that a viscosity-controlling effect is exhibited by the amide group-containing acrylic resin when the aliphatic isocyanate-based active methylene blocked isocyanate used as the curing agent has a mean functional group number of greater than 3, that intermingling and inversion at the interfaces between each of the coating film layers is limited when coating is by a 3-coat, 1-bake method, and also that curing of the intercoating film begins before that of the base coating film and clear coating film, allowing a sufficient flow property to be ensured, and a product results that has an excellent ground layer masking property against roughening of the electrodeposition coating, such that a multilayer coating film is obtained that has an excellent finished appearance and excellent coating properties, and especially chipping resistance.
PTL 2 describes an aqueous coating composition for a second aqueous coating material, for formation of a multilayer coating film by application of a first aqueous coating material, a second aqueous coating material and a clear coating material in that order in a wet-on-wet manner. The aqueous coating composition comprises (a) 40 to 60 parts by mass of an emulsion resin obtained by emulsion polymerization of a monomer mixture containing 0.5 to 10 mass % of a polyfunctional vinyl monomer (amount with respect to the total monomer component), a carboxyl group-containing vinyl monomer, a hydroxyl group-containing vinyl monomer and another vinyl polymerizable monomer, (b) 1 to 5 parts by mass of an amide group-containing water-soluble acrylic resin, (c) 5 to 20 parts by mass of a urethane emulsion and (d) 15 to 35 parts by mass of a curing agent, in 100 parts by mass of resin solid content in the coating composition.
According to PTL 2, due to the crosslinked structure in the emulsion particles, coating in a wet-on-wet manner causes the clear coating material component to infiltrate into the lower layer coating film, or in other words, formation of a mixed layer between the lower layer coating film and upper layer coating film is inhibited, and as a result, the energy required during formation of the multilayer coating film is lowered and a multilayer coating film with an excellent outer appearance and water resistance can be obtained. PTL 2 also teaches that the second aqueous coating material has excellent storage stability, because it comprises (b) an amide group-containing water-soluble acrylic resin, (c) a urethane emulsion and (d) a curing agent.
PTL 3 describes a coating method in which an aqueous primer, aqueous base coat material and clear coating material are coated onto a plastic base material and the three layers are simultaneously baked at a temperature of no higher than 100° C., wherein the aqueous primer comprises an aqueous polyolefin-based resin and an aqueous acrylic-based resin, and the aqueous base coat material comprises an aqueous polyurethane resin, a hydroxyl group-containing aqueous acrylic resin and/or an aqueous polyester resin and a melamine resin, the melamine resin being a butyl/methyl mixed etherified melamine resin and having a weight-average molecular weight in the range of 1500-3000, the clear coating material comprising a hydroxyl group-containing resin and an isocyanate crosslinking agent, and the isocyanate crosslinking agent including a polyisocyanate compound with a urethodione structure and a trimer or greater diisocyanate compound, whereby a multilayer coating film with excellent adhesion, water resistance and durability can be formed.
PTL 4 describes a method of forming a multilayer coating film in which an aqueous intercoat material is coated onto a base material having both a steel sheet and a plastic base material to form an intercoating film, an aqueous base coating material is coated onto the formed intercoating film to form a base coating film, an organic solvent-based clear coating material is coated to form a clear coating film, and the three layers: intercoating film, base coating film and clear coating film are heated for curing, the method of forming a multilayer coating film being characterized in that the aqueous base coating material contains, in 100 mass % of resin solid content, (a) 10 to 60 mass % by solid content of an acrylic resin emulsion obtained by emulsion polymerization of a monomer mixture containing 0.2 to 20 mass % of a crosslinkable monomer, (b) 5 to 40 mass % by solid content of a water-soluble acrylic resin, (c) 20 to 40 mass % by solid content of a melamine resin and (d) 10 to 40 parts by mass of a propylene glycol monoalkyl ether with respect to 100 parts by mass of the coating resin solid content, whereby the outer appearance of the steel sheet and plastic base material become uniform.